1. Field of the Invention
The present invention relates to a zoom lens system which is to be used with compact cameras and so on, has a short back focal length and uses a negative lens unit disposed on the image side therein.
2. Description of the Prior Art
Lens systems for compact cameras which need not have long back focal lengths, unlike lens system for single-lens reflex cameras, may use nagative lens units disposed on the image side therein for shortening total lengths of the lens systems. For each of the lens systems which use the negative lens units disposed on the image side therein, it is conventional to correct aberrations by composing the negative lens unit of a positive lens element and a negative lens element which have refractive powers contrary to each other.
Since aberrations are varied remarkably by zooming a zoom lens system which has a high vari-focal ratio, it is necessary for such a zoom lens system to correct aberrations more strictly in each of lens units used for composing the zoom lens system.
When chromatic aberration is corrected by strengthening a refractive power of a positive lens element and a refractive power of a negative lens element, it is impossible to correct the other aberrations, astigmatism and coma in particular. As a conventional example which uses a positive lens element and a negative lens element for composing the above-mentioned negative lens unit in addition to an aspherical surface adopted as a surface of the positive lens element for balancing chromatic aberration with the other aberrations, there is known a zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 3-127,008. The correcting means adopted for this conventional example and the similar zoom lens systems is effective for favorably correcting astigmatism and coma, but since the positive lens element has a weak paraxial refractive power, the zoom lens systems allow longitudinal chromatic aberration to be varied by moving zooming lens units and have chromatic aberration which cannot be said to be sufficiently corrected.
A zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 3-45,916 is known as a conventional example in which the above-mentioned negative lens unit is composed of a plurality of negative lens elements and a plurality of positive lens elements for correcting chromatic aberration. This zoom lens system is composed of three positive, positive and negative lens units or four positive, negative, positive and negative lens units.
In addition to the conventional examples described above, a zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 3-233,422 is known as a zoom lens system which comprises an image side negative lens unit using a cemented lens component for correcting chromatic aberration. This zoom lens system is composed of three negative, positive and negative lens units.
Out of the conventional examples mentioned above, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 3-45,916 has a defect that it exhibits a low effect for correcting aberrations other than chromatic aberration due to a fact that a front subunit of such a negative lens unit as the third lens unit out of the three lens units or the fourth lens unit out of the four lens units has a remakably low refractive power when the front subunit is composed of a cemented lens component consisting of a negative lens element and a positive lens element or a positive lens element and a negative lens unit, and a rear subunit of the negative lens unit is composed of a negative lens component.
Further, when a third lens unit is composed of a cemented lens component consisting of a positive lens element, a negative lens element and a positive lens element in a zoom lens system comprising three lens units, like the zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 3-233,422, the cemented lens component has a strong negative refractive power and produces aberrations, thereby making it necessary to dispose a positive lens component on the object side for cancelling the aberrations and use a large number of lens elements for composing the zoom lens system while enhancing a manufacturing cost thereof.